Although risk-taking behavior, including substance use and abuse, unprotected sex, delinquent activity, and reckless driving, is a primary cause of physical and psychological problems in adolescence, interventions designed to reduce teenagers' risky behavior have proven largely ineffective. This application describes a series of experiments designed to examine the neural underpinnings of age differences in the impact of peer influence on risk-taking. Previous research has shown that the presence of peers increases risk-taking among adolescents, but not adults. In the proposed research, individuals in three age groups (adolescents, young adults, and adults) will be administered a series of computer-based risk-taking tasks, as well as a series of tasks assessing reward salience, preference for immediate versus delayed rewards, planning, and response inhibition while neural activity is assessed using fMRI. Individuals will come to the imaging facility with two friends of their choosing and will be situated in a room with the capacity for simultaneous computer display of the subject's task performance. Using a within-subjects design, the presence or absence of peers' knowledge of the participant's task performance will be manipulated experimentally and disclosed to the subject. Individuals' behavior and neural activity when performing the tasks while their friends are observing them will be compared to their behavior and activity while performing the tasks alone. Specific aims are to (a) test the hypothesis that disruptive impact of peer influence on risk-taking and decision-making is stronger during adolescence than adulthood; (b) test the hypothesis that patterns of brain activation observed among adolescents engaged in risk-taking tasks show relatively more engagement of socio-emotional brain systems and relatively less engagement of cognitive control brain systems, when peers are present than when peers are absent, but that the relative engagement of these systems among adults will not differ as a function of the peer context; and (c) examine the potential mediating roles of reward salience, reward immediacy, planning, and response inhibition in accounting for these age differences. The proposed research has the potential to inform the designing of more effective interventions to reduce adolescent risk-taking by expanding our understanding of the mechanisms through which teenagers are influenced by their friends to engage in health-compromising behavior. [unreadable] [unreadable] [unreadable]